1. Field of the Invention
The present invention relates to a lighting circuit capable of coping with short-circuiting of and current leakage from, a discharge lamp.
2. Description of the Related Art
Recently, greater attention is being paid to a compact discharge lamp (a metal halide lamp or the like) as a light source for vehicular lamps. As shown in FIG. 11, for example, a known lighting circuit a includes a DC power supply b, a switching power supply circuit c, and a DC-AC converter d.
In this lighting circuit a, a DC voltage, which is acquired by the switching power supply circuit c based on the voltage from the DC power supply b, is converted by the DC-AC converter d to an AC voltage of a rectangular wave, which is in turn supplied to a discharge lamp f via a current-limiting inductive element e.
FIG. 12 shows a lighting circuit g as a known lighting system, which supplies an AC voltage of a negative rectangular wave to a discharge lamp. The lighting circuit g comprises a DC power supply b, a switching power supply circuit h and a DC-AC converter i. In this circuit g, the switching power supply circuit h and the DC-AC converter i generate a negative rectangular wave based on the voltage from the DC power supply b, and this rectangular wave is then supplied to the discharge lamp f via the current-limiting inductive element e.
When one electrode of a discharge lamp contacts a conductive member or the like to cause short-circuiting or the electrodes are wetted with water or the like, resulting in a current leakage, the conventional lighting circuits are likely to suffer increased power loss, which will heat the circuit.
With the discharge lamp f having connector terminals ja and jp as shown in FIG. 11, for example, when a current leakage occurs on the jb side in the lighting circuit a, the output current is separated into a lamp current IL and a leak current Ir as indicated by the arrows.
Although the lighting circuit a tries to perform power control on the discharge lamp based on the lamp current IL of the discharge lamp f or its equivalent current detected (e.g., the lamp current is detected by means of a detecting resistor k provided between the switching power supply circuit c and the DC-AC converter d) at that time, some of the power supplied to the discharge lamp from the switching power supply circuit c leaks out in the phase where the potential at the connector terminal ja is low and the potential at the connector terminal jb is high, the lamp current decreases or the discharge lamp becomes off (IL=0), or the lamp voltage drops (to a level equal to the product of the leak current Ir and leak impedance Z or the product of the lamp current and lamp impedance) and, at the same time, the leak current Ir is not detected. When control is carried out to ignite the discharge lamp f on, for example, the rated power, therefore, the power that is output from the lighting circuit a becomes greater than the rated one due to the leak current.
In the lighting circuit g in FIG. 12, when a current leak occurs, the leak current Ir flows into the lighting circuit from the ground through the leak impedance Z and the leak current flows across a detecting resistor k' provided between the switching power supply circuit h and the DC-AC converter i. This may cause erroneous detection at the detecting resistor k' that is provided to detect the lamp current, so that the discharge lamp f, even in an off state, is erroneously determined as if it is on, as a result of detecting the current that includes the leak current.
One possible way of overcoming this shortcoming is to detect the lamp voltage or its equivalent signal, pass it through a low-pass filter and inhibit the operation of the lighting circuit when the lamp voltage drastically varies.
While this scheme is effective when the discharge lamp becomes off at the time the current leaks in the lighting circuit a in FIG. 11, however, the scheme still has a difficulty in detecting current leakage when the discharge lamp does not become off upon occurrence of current leakage unless the detection sensitivity is increased considerably by properly setting the time constant of the low-pass filter. It is therefore hard to distinguish between the normal lighting of the discharge lamp and the state of the discharge lamp at the time current leakage occurs. The lighting circuit g in FIG. 12 has a similar shortcoming besides a difficulty in deciding the reference level for determining at which pulse width of the detection signal, obtained in response to a drastic variation in lamp voltage, the operation of the lighting circuit should be stopped.
In a case where a drastic variation occurs in the voltage from the DC power supply b, depending on the frequency of the variation, the lamp voltage or the lamp current may show a change similar to what occurs at the time of current leakage, even when short-circuiting of the discharge lamp or current leakage has not occurred. At this time, erroneous detection may happen.
Accordingly, it is an object of this invention to reliably detect short-circuiting of, or current leakage from, a discharge lamp.